I. Field of the Invention
The present invention relates to an improved check valve for use in positive displacement compressors and in other applications requiring pressure actuated, unhindered flow in one direction and rapid and complete blockage of flow in the opposite direction. In positive displacement compressors, these valves control the flow of gas into and out of the compression chamber and are usually called suction and discharge valves.
II Prior Art
Conventional suction and discharge valves are pressure actuated, spring loaded check valves containing one or more valve flow control elements. These elements cover flow passages in the valve structure when shut, preventing back flow. When a pressure differential is established in the direction of intended flow, the pressure differential lifts the elements from the flow passage, permitting flow therethrough. Valve spring force is slight and exerts little influence on valve opening.
Various types of valve flow control elements are known for use in compressor valves in common use. A partial list includes: plate, poppet, reed and strip elements. Regardless of type, when the valve is closed, the area of the flow control element exposed to the upstream gas pressure is smaller than the flow control element area exposed to the downstream pressure. This is so because only that portion of the upstream side of the flow control element facing into the flow passage is in contact with the upstream gas. The remaining upstream surface area of the flow control element is in contact with the valve structure adjacent to the flow passage. This area of contact forms the seal to prevent back flow and provides mechanical support for the flow control element when an opposite pressure differential in the direction of the downstream face is present. The whole of the downstream face is exposed to the downstream pressure when the valve is shut. Opening of these pressure actuated valves occurs when the force of the upstream gas pressure operating on the smaller upstream exposed area of the flow control element exceeds the force of the downstream pressure operating on the whole downstream area of the flow control element. The opening pressure differential of such compressor valves therefore depends primarily on the operating pressure of the compressor.
An opening pressure differential of approximately ten percent of the upstream pressure is typically observed with conventional compressor valves. The large pressure imbalance present upon opening causes a high initial flow velocity through the valve. The Bernoulli flow formula predicts a velocity of thirty-eight percent of the speed of sound with a ten percent pressure drop. The almost explosive initial flow causes abrupt pressure changes within the compression chamber and adjacent gas passages that triggers the intense pressure oscillations which can be detected throughout the suction and discharge events. These pressure oscillations increase the work of compression. Also, the variations in gas density associated with the pressure oscillations and their phase angle at valve closing have an effect on the amount of gas delivered. The differential area relationship of the conventional compressor valve thus adversely influences two aspects of compressor performance, mechanical efficiency and predictability or consistency of throughput.